The PR for changing how CSS and JS is included only refers to the Helium theme. There is no mention of other themes, and the implication is that other themes do not support this functionality.
If custom CSS and JS can no longer be loaded without using the Helium theme, this seems to mean that other themes are now effectively useless. If this is the case I think you should just get rid of Theme as I cannot see it serves any purpose. If this is not the case, loading CSS and JS without using Helium should be documented.
The PR for changing how CSS and JS is included only refers to the Helium theme. There is no mention of other themes, and the implication is that other themes do not support this functionality.
If custom CSS and JS can no longer be loaded without using the Helium theme, this seems to mean that other themes are now effectively useless. If this is the case I think you should just get rid of
Themeas I cannot see it serves any purpose. If this is not the case, loading CSS and JS without using Helium should be documented.